The transmission of various types of digital data between computers continues to grow in importance. The predominant method of transmitting such digital data includes coding the digital data into a low frequency base data signal and modulating the base data signal onto a high frequency carrier signal. The high frequency carrier signal is then transmitted across a network cable medium, via RF signal, modulated illumination, or other network medium, to a remote computing station.
At the remote computing station, the high frequency carrier signal must be received and demodulated to recover the original base data signal. In the absence of any distortion of the carrier signal across the network medium, the received carrier would be identical in phase, amplitude, and frequency to the transmitted carrier and could be demodulated using known mixing techniques to recover the base data signal. The base data signal could then be recovered into digital data using known sampling algorithms.
However, the network topology tends to distort the high frequency carrier signal due to numerous branch connections and different lengths of such branches causing numerous reflections of the transmitted carrier. The high frequency carrier is further distorted by spurious noise caused by electrical devices operating in close proximity to the cable medium. Such problems are even more apparent in a network which uses home telephone wiring cables as the network cable medium because the numerous branches and connections are typically designed for transmission of plain old telephone system POTS signals in the 0.3–3.4 kilohertz frequency and are not designed for transmission of high frequency carrier signals on the order of 7 Megahertz. Further yet, the high frequency carrier signal is further distorted by turn-on transients due to on-hook and off-hook noise pulses of the POTS utilizing the network cables.
Such distortion of frequency, amplitude, and phase of the high frequency carrier signal degrades network performance and tends to impede the design of higher data rate networks and challenges designers to continually improve modulation techniques and data recovery techniques to improve data rates. For example, under the HPNA 1.0 standard, a 1 Mbit data rate is achieved using pulse position modulation (PPM) of a carrier, while the more recent 2.0 standard achieves a 10 Mbit data rate using a complex modulation scheme utilizing a frequency diverse quadrature amplitude modulation (QAM).
A problem associated with advancing standards and increasing data rates is that, as in the HPNA example, the modulation techniques are not the same. As such, backwards compatibility is not inherent in the design of the newer systems. For example, in the HPNA system, to be backwards compatible, the newer 2.0 receiver must be able to demodulate both the PPM modulated carrier compliant with the 1.0 standard and the frequency diverse QAM modulated carrier compliant with the 2.0 standard. As such, many of the functions in the receiver must be implemented in two distinct circuits, one circuit for the PPM and one circuit for the QAM, thereby increasing the cost and complexity of the receiver.
For example, receivers typically include an A/D converter for sampling the modulated carrier signal and generating a series of samples occurring at a sample frequency. The series of samples are input to the remainder of the receiver circuitry that is typically implemented on a digital signal processor (DSP).
An amplifier conditions the signal, in accordance with a gain setting of the amplifier, prior to the A/D converter to assure that the signal parameters are within the dynamic range of the A/D converter. The gain of the amplifier is typically set using a closed loop feedback system. A problem exists in that a PPM modulated carrier signal and a QAM modulated carrier signal have significantly different power envelopes. More specifically, the power on the PPM modulated carrier occurs in short pulses on the order of 3 usec and the power on a QAM modulated carrier is continuous for the entire transmission frame. Feedback circuitry useful for setting input gain in a pulsed power environment is fundamentally different than feedback circuitry useful in setting input gain in a continuous power environment. As such, complicated feedback circuitry would be required to accommodate rapid gain adjustment for both PPM and QAM signals. Such circuitry would significantly add to the size and cost of a receiver.
Therefore, based on recognized industry goals for size and cost reductions, what is needed is a device and method for adjusting input gain for an amplifier in a receiver capable of receiving modulated carrier signals modulated using multiple modulation techniques.